From DE 10 2009 045 510 A1 a device for varying the swept volumes of a first hydraulic machine and of a second hydraulic machine is known. A piston of a piston-cylinder device is functionally connected to axes of the hydraulic machines. Depending on a power demand from the driver, the piston can be acted upon by way of a position-regulating valve unit and a high-pressure-regulating valve unit of a valve device or control valve device, in the area of a functional surface that delimits a piston space, with pressures that correspond to a hydraulic pressure present in the area of the hydraulic machines and that act in a first adjustment direction of the axes of the hydraulic machines. The pressure in the area of the hydraulic machines can be adjusted in a controlled manner by means of the position-regulating valve unit and can be limited in a controlled manner by means of the high-pressure-regulating valve unit. Furthermore, the piston can be acted upon by way of the valve device in the area of a further functional surface that delimits a further piston space, with pressures that correspond to a hydraulic pressure present in the area of the hydraulic machines and that acts in a second adjustment direction of the axes of the hydraulic machines, whereby the mode of operation of the position-regulating valve unit in the area of the high-pressure-regulating valve unit can be reversed.
Characteristic for a transmission constructed with the hydraulic machines and the associated device described above is the possibility of continuously variably adjusting the overall transmission ratio, and load-sensitivity. Both characteristics are achieved with the help of the hydraulic machines designed as axial piston machines. The transmission ratio of the transmission is varied continuously by pivoting axes of the hydraulic machines, this taking place by virtue of the position-regulating operating mode in the area of the device. Load-sensing is implemented in the high-pressure-regulating operating mode. A maximum high pressure in the area of the hydraulic machines can be limited by a control pressure that can be applied in the area of the device. If the high pressure demanded by the hydrostat or the hydraulic machines, for example due to an external torque as when a wheel loader is driven onto a pile of rubble, is larger than the maximum pressure permitted by the control system, the swept volumes of the hydraulic machines are changed by pivoting the axes during traction operation of a vehicle drive-train made with the transmission and by an associated reduction of the reciprocal transmission ratio, which corresponds to a brief pivoting of the hydrostat.
In the position-regulating operating mode an armature of a proportional magnet of the position-regulating valve unit is drawn into a solenoid by applying an actual current regulated by a current regulator. By virtue of the magnetic force produced, a slide of the position-regulating valve unit made as a 4/2 control valve is displaced. In the area of a feed line the position-regulating valve unit is connected to the low pressure of a substantially unpressurized zone or tank area. In the area of a further feed line the high pressure acting in the area of the hydraulic machines is applied to the position-regulating valve unit. The position of the valve slide of the position-regulating valve unit defines whether the low pressure or the high pressure is passed on in the direction of the high-pressure-regulating valve unit.
The position of the valve slide of the high-pressure-regulating valve unit is determined by an overall force component acting on the valve slide, which is composed of the spring force of a spring device and the pressures acting in the area of functional surfaces of the valve slide. In this case the high pressure acting in the area of the hydraulic machines is applied in the area of a first functional surface of the valve slide of the high-pressure-regulating valve unit. In addition, a control pressure can be applied to another functional surface of the valve slide of the high-pressure-regulating valve unit, this control pressure being variable, for example, by means of a control pressure valve current.
In the position-regulating operating mode the valve slide of the high-pressure-regulating valve unit is displaced by the spring force to its fully out-of-the-way end position, so that the piston of the piston-cylinder device is moved exclusively as determined by the pressure in the area of the position-regulating valve unit. The movement of the piston of the piston-cylinder device is relayed back to the position-regulating valve unit by a mechanical feedback mechanism, whereby the valve slide of the position-regulating valve unit moves to a stable position.
During the high-pressure-regulating operating mode, the valve slide of the high-pressure-regulating valve unit is displaced by the pressures acting in the area of the functional surfaces in opposition to the spring force of the spring device, to its second out-of-the-way end position, and the operating mode of the position-regulating valve unit is reversed.
To be able to adjust the swept volumes of the two hydraulic machines to the desired extent, it has to be ensured by way of a complex actuation logic that in relation to the valve slide of the high-pressure-regulating valve unit, the valve slide of the position-regulating valve unit adopts over the full operating range the appropriate positions in each case for the functioning of the device, in which the controlling hydraulic fluid volume flow triggers the desired system reaction and, for example, an undesired restriction of the dynamics or a reversal of the hydraulic fluid volume flow are avoided.
The complex logic provided for determining the appropriate valve setting of the position-regulating valve unit takes into account both measurable operating parameters of a transmission made with the hydraulic machines that form a hydrostat, and also transmission-internal parameters such as a current overall transmission ratio, internal gear ratios, and the rotational speed of the drive output.
In this, however, it is problematic that to obtain the desired or necessary system reaction in each case, very severe demands are imposed on the logic system in relation to robustness and speed.